Safety belts are effective only if they succeed in protecting the occupants of a vehicle from being thrust against interior parts of the vehicle while keeping the forces exerted on the occupants' bodies within tolerable limits. Although an enlargement of the interior space of a vehicle may be a solution to the problem, a satisfactory increase in size is often economically impractical.
The forces exerted by safety belts may be limited by using expansion (elastic) elements of the type disclosed in the French Patent No. 1,180,364, but such an arrangement leads to an increase in the forward displacement of the vehicle occupants during rapid deceleration. The use of elastic elements must therefore be accompanied by an enlargement of the vehicle interior.
Another possibility for improving safety belt effectiveness is to tighten the belts (within tolerable tension limits) at the moment that the restraining force of the belts is needed. Currently known automatic belt winders with centrifugal weights are unsatisfactory for this purpose, however, particularly when the vehicle is involved in a multiple impact accident or overturns. This type of belt tensioning device releases the safety belts after tightening thus defeating the desired restraining action.